Chapter 3 Refrigerants - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

Mechanical Engineering SSC JE (Technical)

Mechanical Engineering : Chapter 3 Refrigerants - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev

The document Chapter 3 Refrigerants - RAC (Refrigeration and Air Conditioning), Mechanical Engineering Mechanical Engineering Notes | EduRev is a part of the Mechanical Engineering Course Mechanical Engineering SSC JE (Technical).
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Chapter 3

REFRIGERANTS

Compressors for Different Refrigerants

RefrigerantCompressor
NH3Reciprocating & screw compressor
R=11Centrifugal compressor
R-12Reciprocating compressor
R-22Reciprocating compressor
R-113Centrifugal compressor
R-114Rotary compressor

Applications of Refrigerants

RefrigerantApplication
NH3Cold storage, ice plants, refrigerators
H2OWater, Li Br absorption system
CO2Used as dry ice in transport
R-11Central air conditioning
R-12Domestic refrigerator, water cooler
R-22Window type air conditioner
R-113Air conditioning

DESIGNATION OF REFRIGERANTS
Saturated Hydrocarbons

  •  If the compound is CmHnFpClq
    n + p + q = 2m + 2

then,

designation is R(m - 1) (n + 1) P

Eg. CCI3 is R-11

  •  The brominated refrigerants are denoted by and additional B and a member to denote as to how many chlorine atoms are replaced by bromine atoms

Eg. CF3 Br is R13 B1


Unsaturated Compounds - CmHnFpClq
where n + p + q = 2m
they are designated by R (m-1) (n+1)P
Eg. C2H4 is R1150

Inorganic Refrigerants
Designation is R (700 + molecular weight)
Eg. NH3 is R 717, CO2 is R744

Basic properties of Some important Refrigerants

CFC 12 or R-12-or CCl2F2

  •  Normal boiling point (N.B.P.) = 29.8ºC
  •  It is used in domestic refrigerators, car air conditioners, water coolers etc.
  •  It is used with reciprocating compressor
  •  Because of low value of g. discharge temperature is low. so it is very suitable for hermetically sealed units

CFC-11 or R-11 or CCl3F

  •  N.B.P. -23.7ºC
  •  Having more boiling point, It has vacuum in the evaporator.
  •  It is used with centrifugal compressor.
  •  Its application is in central air conditioning.

HCFC-22 or R-22 or CHClF2

  •  N. B. P. – 40.8ºC
  •  It is used in window air conditioning.
  •  It is used with reciprocating compressor.
  •  It has high g value, so discharge temperature is high. Thus it is not very suitable for hermetically sealed unit for more than one year.
  •  N.B.P. of commercially used refrigerants vary from -50ºc to + 50ºC.

PROPERTIES OF GOOD REFRIGERANTS

  •  Low boiling point
  •  Evaporating pressure should be low and positive and near to atmospheric pressure
  •  High critical temperature
  •  Low critical pressure
  •  Low freezing point
  •  Non flammable, non explosive
  •  Either completely miscible or completely immiscible with oil
  •  Non corrosive
  •  High dielectric strength
  •  High thermal conductivity
  •  Low viscosity
  •  Low specific heat
  •  Low surface tension
  •  Low cost

SECONDARY REFRIGERANTS

  •  They are used to reduce the quantity of main refrigerant and to reduce pressure losses. They are cheap in cost.

Eg. brines such as : CaCl+ Water & NaCl + water, Ethylene glycol + water and propylene glycol + water.

  •  CaCl2 and NaCl is mixed with water to reduce the freezing point of water.

OZONE DEPLETION 

  •  Ozone layer is in stratosphere.
  •  It protects humans from UV rays which can cause skin cancer.
  •  Chlorine atoms in CFC causes ozone depletion.
  •  Hydro-Chloro-flouro-carbons (HCFCs) have low ozone depletion tendency because of less chlorine.

But they have Global warming tendency.

  • Pure hydro carbon is highly inflammable due to hydrogen atoms.
  • Azerotrope: These are non ideal mixtures of two or more refrigerants behaving as pure substance having close N.B.P. Their bubble point temperature is equal to dew point temperature.
  • Bubble temperature is the temperature at which liquid mixture starts boiling.
  • Dew point temperature is the temperature at which vapour mixture starts condensing.
  • CO2 refrigeration system has very low COP because critical temperature of CO2 is very low and very close to condensing temperature.
  • Because of flammability & toxicity, ammonia is not used in domestic refrigerator, comfort air conditioning.
  • If water content in refrigerant is more than that can be dissolved by the refrigerant, there is a danger of ice formation and consequently choking of capillary tube used in throttling system. This is moisture chocking.
  • Silica gel is used in liquid line for dehydration of refrigerant.
  • Miscibility of refrigerants with oil
RefrigerantMiscibility with oil
R-11, R-12Good miscibility
NH3, CO2No miscibility
R-22Partial miscibility
  •  In evaporator, oil separates from refrigerant vapour. Accumulation of oil in evaporator called oil choking.
  •  In immiscible oil refrigerant mixture, oil separator is used after compressor.
  •  Ammonia corrodes copper. So iron and steel are used with its system.
  •  Refrigerants in decreasing order of normal boiling point.

R-11(–23.7ºC) > R-12(-290C) > NH3(–33.30C) > R-22(–410C) > CO2(– 73.60C)

  • Refrigerants in increasing order of freezing point 

CO2 < NH3 < R-11 < R-12 < R-22

  • Refrigerants in decreasing order of critical temperature

R-11 < NH3 < R-12 < R-22 < CO2

  • Refrigerants in decreasing order of Critical Pressure

NH3(113.86) > CO2(73.8) > R-22 > R-11 > R-12(41.2)

  • Refrigerants in decreasing of COP

R -11 > NH3> R-12> R-22> CO2

Leakage Detection of Refrigerants

LeakageDetected By
NH3Burning sulphur candle
HalocarbonHalide torch test
Hydrocarbon and PropaneSoap and water test
SO2NH3 swab test
Refrigerationelectronic leak detector
Equipment(high sensitivity)
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